Standard dual mode filters do not realize all canonical bridge couplings. Also, these standard filters have large bridge coupling dispersion which can cause difficulty in structures with external bridges. In working to overcome this shortcoming, a canonical dual mode filter was developed and described in U.S. Pat. No. 4,060,779.
This patent concerns a plural cavity waveguide filter comprising a plurality of cascaded waveguide cavities each resonating in first and second independent orthogonal modes. The cavities may be either square and/or circular resonating in the TE.sub.101 or TE.sub.111 modes, respectively, and it is possible to realize the general class of coupled cavity bandpass transfer filter functions by providing that the input and output couplings to the filter be physically connected to the first physical cavity.
Although both the input and output filter ports are connected to the same physical cavity, these ports are coupled to different orthogonally-oriented electrical cavities within the same physical cavity. Although, theoretically the design would seem to be complete, it has been found that certain distinct disadvantageous characteristics are present in the operation of the canonical dual mode filter described in the above-noted patent.
The main disadvantage of the prior art, exemplified for example by the above-referred to patent, lies in the difficulty of minimizing the leakage path between the input and output ports. Such leakage degrades the usefulness of the filter and adversely affects the ability to provide a generally desired symmetrical out-of-passband response. It should, therefore, be evident that an improved technique used in reflective dual mode filters to minimize the leakage path and symmetize the out-of-band rejection characteristic, would constitute a significant advancement in the art.